Since some cyclic phosphotriesters hydrolise at a suitable rate by nucleophilic buffers, the neutral phosphotriester may be a good transport form to facilitate the uptake of nucleotide antimetabolite. This proportion, first proposed by us for cyclic AMP transport, will be applied to ara-C phosphate. The chemical synthesis involves the reaction of N", 2', and 3'-triacetyl ara-C with 2-chloro-1,3,2-dioxaphospholane followed by treatment with sulfur in order to produce the phosphorothionate. The target compounds is 5'-0-(2-thio-1,3,2-dioxaphospholanyl)ara-C. The compound will be characterized by PMR, 31P-NMR and mass spectroscopy. Hydrolytic stability of the cyclic ethylene group will be determined in bicarbonate buffers and blood serum. Biological activity of the phosphotriester will be compared with ara-C in Chinese hamster ovary cells and ara-C resistant P-385 cells in culture. If the rate of biotransformation is suitable, one would obtain ara-C-5'-phosphate within the cells, there would be no need for kinase action and one cause of resistance would be eliminated.